The purpose of this research is to characterize the effects of viral respiratory disease during early life on postnatal lung growth and development. Parainfluenza (Sendai) virus infection in rats is being used as an experimental model of viral bronchiolitis and pneumonia in human infants. Studies on viral respiratory disease in infants have shown that viral infection in the first 2 years of life is often associated with persistently abnormal subsequent lung function and a greater predisposition to chronic lung disease. The proposed research will determine the pathologic and morphometric changes that occur in lungs of young rats infected when the lung is rapidly growing. The research will also examine virologic and immunologic mechanisms that may play a role in making young animals more susceptible to viral lung damage during early life. Quantitative morphology studies will focus in detail on ultrastructural morphometric alterations in postnatal growth of alveolar parenchyma that may decrease the diffusion capacity of exchange tissue in viral-infected infants. Morphometric studies on airway corrosion casts of viral-infected infant rats will focus on viral-induced changes in airway growth that may contribute to obstruction of conducting airways. Viral-induced changes in lung compliance will be assessed by static saline pressure-volume physiologic studies.